In recent years, terminal devices represented by a PHS (Personal Handy phone System), a mobile telephone and the like have become popular, allowing users to make calls and obtain information anywhere and anytime. In particular, lately, as the amount of available information has shown a steady growth, high-speed and high quality wireless communication systems have been introduced to enable downloading a mass of data.
In such wireless communications, there is a need to duplex signal for transmission/reception. The duplexing of systems may typically include TDD (Time Division Duplex) for switching transmission/receipt by means of time division and FDD (Frequency Division Duplex) for duplexing transmission/receipt using different frequencies. In addition, multiple access systems which allow simultaneous communication with a plurality of terminal devices may typically include TDMA (Time Division Multiple Access) for switching a plurality of terminal devices by means of time division, FDMA (Frequency Division Multiple Access) for dividing a frequency band, and CDMA (Code Division Multiple Access) for multiplying different signs of respective terminal devices.
For example, the next generation PHS communication standards for high-speed digital communication may include ARIB (Association of Radio Industries and Business) STD T95 (Non-Patent Document 1) and PHS MoU (Memorandum of Understanding), which have employed OFDMA/TDMA TDD Broadband Wireless Access System (next generation PHS system).
OFDMA (Orthogonal Frequency Division Multiple Access) refers to multiple access in OFDM (Orthogonal Frequency Division Multiplexing). OFDM is an advanced version of FDM (Frequency Division Multiple), which is a system using a frequency band effectively by dividing a carrier signal into a plurality of sub carriers on a frequency axis and making phases of signal waves orthogonal between adjacent sub carriers to partially overlap bands of the sub carriers. While one terminal device occupies all sub carriers in OFDM, OFDMA forms sub channels by grouping a plurality (e.g., 24) of sub carriers and allows multiple access by sharing all the sub channels between a plurality of terminal devices. For example, a sub channel divides a frequency band of 18 MHz into 20 sub frequency bands.
In the meantime, the next generation PHS system allows multiple access by TDMA in addition to OFDMA. TDMA is a system which divides a frequency into a plurality of time slots on a time axis and conduct communication with a plurality of parties. In the present conditions, it is assumed to divide an up link (from a terminal device to a base station) and a down link (from a base station to a terminal device) into four sub links respectively. That is, in the next generation PHS system, both the frequency axis and the time axis can be sub-divided into communication blocks which are dynamically assigned to a plurality of terminal devices, thereby allowing efficient communication. A communication block defined by one time slot in one sub channel is referred to as PRU (Physical Resource Unit) and it is assumed to use 80 PRUs in the vicinity of one base station. Also, a communication block defined by one time slot in one sub channel is referred to as PRU (Physical Resource Unit) and it is assumed to use 36 to 40 PRUs per one base station.
While a base station can use 20 sub channels, as described above, one of these sub channels is used as a control channel (CCH) and the remaining sub channels are dynamically assigned to terminal devices (Dynamic Channel Assign (DCA)). An anchor channel or an extra channel is assigned to PRU included in a sub channel used for communication. The anchor channel is assigned by one to each terminal device and includes a map of PRU to which an extra channel for the terminal device is assigned. The extra channel is a channel which actually includes data and a plurality of extra channels are assigned to one terminal device depending on the amount of data and communication situation. Notification of the assignment of extra channels by a map included in the anchor channel in the same manner is called “FM-mode” (Fast access channel based on Map-Mode).
The anchor channel is assigned to the PRU having the best communication quality found when the carrier sense is performed for all PRUs. Although the extra channel basically does not perform carrier sense, if a PRU in which no communication is conducted is newly used by their base station, the extra channel is assigned to the PRU after carrier sense is performed. In this manner, since a base station can dynamically change the position and number of extra channels through the anchor channel, it is possible to transmit/receive a mass of data at high speed.
However, a PRU in the OFDMA system has a problem in that it is likely to be interfered with adjacent PRUs. Many techniques have been proposed for avoiding such interference with wireless communications. For example, Patent Document 1 discloses a technique in which a down link frame is divided into similar sized resource blocks, transmission data is scheduled from the beginning of the respective resource blocks, and data beyond the capacity of the resource blocks is scheduled to be transmitted at the end of the resource blocks assigned to other sectors. This technique states that it is possible to prevent communication from being continuously conducted in a co-channel sector and reduce co-channel interference.
Also, a PRU in the OFDMA system has a problem in that it is likely to be interfered with adjacent PRUs in a frequency direction. Many techniques have been proposed for avoiding such interference with wireless communications. For example, Patent Document 1 discloses a technique in which a down link frame is divided into similar sized resource blocks, transmission data are scheduled from the beginning of the respective resource blocks, and data having capacity beyond the resource blocks are scheduled to be transmitted at the end of the resource blocks assigned to other sectors. This technique states that it is possible to prevent communication from being conducted with delay in a co-channel sector and reduce co-channel interference.    [Patent Document 1] JP-T-2006-515141 (the “JP-T” as used herein means a published Japanese translation of a PCT patent application)    [Non-Patent Document 1] ARIB (Association of Radio Industries and Business) STD-T95